The present invention relates to terminating electrical transmission lines. In particular, the present invention relates to terminating transmission lines with a matched resistance.
In communication systems, and within complex electronics systems, electrical signals are transmitted from one location to another over transmission lines. Every transmission line has an inherent impedance that affects the electrical signal. To reduce reflectance of the signal, it is known to match the impedance of the transmission line with a resistance at both the signal source and the signal destination. This resistance is formed by a termination circuit that is connected between the transmission line and a fixed voltage, such as ground or a power supply.
The location of the termination circuit, relative to the transmitter that creates the signal, or the receiver that receives the signal, is critical to properly preventing reflected signals. In particular, the termination circuit should be placed as close as possible to the transmitter or receiver.
When transmitters and receivers are located in an integrated circuit, locating the termination circuit near the transmitter or receiver becomes difficult. It is difficult to place the termination circuit in the integrated circuit itself because resistive materials deposited on an integrated circuit tend to have a large temperature coefficient that causes the resistance of the material to vary widely as the temperature of the circuit changes. Because of these variations, the termination circuit will not always match the impedance of the transmission line.
Alternatively, termination circuits can be placed in a package in which the integrated circuit is seated for connection to the circuit board. However, placing the resistance in the package makes the package "integrated circuit specific", and thus increases the cost of manufacturing.
A termination circuit that can provide a stable resistance and is contained within the integrated circuit is desirable.